Pleckstrin homology (PH) domains contain 100-120 amino acids and are found in over 250 human proteins. About 40 PH domains are known to bind phosphorylated phosphatidylinositide (PtdIns) lipids held in cell membranes. PtdIns phosphorylation and the subsequent binding of PH domain-containing proteins are vital components of signal transduction pathways that regulate cell growth and survival. For example, phosphorylation of PtdIns(4,5)P2 to produce PtdIns(3,4,5)P3 by PtdIns 3-K signals the recruitment and binding of AKT to the inner leaflet of the plasma membrane via recognition of the PH domain. The phosphatidylinositol-3-kinase (PtdIns-3-kinase)/Akt pathway is a survival signaling pathway that is activated in many types of human cancer. Cancer cells are resistant to the mechanisms that cause programmed cell death (apoptosis) in normal cells because they contain these activated survival signaling pathways. The PH domains of proteins, and specifically in this case in Akt, provide novel molecular targets for new types of drugs to prevent and treat cancer.
The PtdIns 3-kinase (PtdIns 3-K)/AKT pathway is of critically importance for cell proliferation and survival. Phosphorylation of PtdIns(4,5)P2 to produce PtdIns(3,4,5)P3 by PtdIns 3-K signals the recruitment and docking of AKT to the inner leaflet of the plasma membrane via its pleckstrin homology (PH) domain. AKT is then phosphorylated at Thr308 by the plasma membrane bound PtdIns dependent kinase-1 (PDK1) and on Ser473 by either intergrin linked kinase (ILK), by the kinase activity of AKT itself or by mammalian target of rapamycin (mTOR)-rictor (TORC2). Once fully phosphorylated, AKT translocates back to the cytosol and nucleus, where it phosphorylates a variety of downstream targets including pro-apoptotic promoters such as forkhead transcription factors FKHR and AFX, as well as the Bcl-2 family member Bad, which is directly inhibited by phosphorylation via AKT. AKT promotes cell survival by activating CREB, and promotes proliferation by activating p70S6 kinase and GSK-3β which contributes to cyclin D accumulation of cell cycle entry. AKT also acts as a mediator for VEGF production and angiogenesis by phosphorylation of mTOR, and defects in the PtdIns 3-K/AKT pathway are found in a variety of cancers, with most abnormalities occurring with mutation events in PTEN. Given the importance of AKT in proliferation and survival signaling, it has the potential to be an important target for cancer drug discovery.
Three genes encode AKT within the mammalian species to produce AKT-1/α, AKT-2/β, and AKT-3/γ isoforms of AKT of which AKT-1 and AKT-2 are expressed throughout the organism while AKT-3 is predominantly expressed in the brain, heart, and kidney. The three isoforms share a high degree of sequence homology within their PH domains but diverge within other regions. However, despite these differences they appear to have similar effects on cellular growth and apoptosis, and these similarities in biological and physiological properties between isoforms coupled with the similarities between their PH domains offers a fortuitous advantage in designing drugs that inhibit all AKT activity.